The invention relates to a method involving the introduction of to-be-treated particulate material into a treatment chamber together with relatively hot gaseous material. The said constituents are charged into a chamber at the top and into a spray means. While dropping freely the sprayed particulate material is passed through a series of zones, one of which is a transition zone. The treated particulate material is collected by the fluidized bed provided in the lower part of the chamber. The fluidized bed is also actuated by a hot gas. Since not only the gaseous medium of the fluidized bed but also the gas entering with the material at the upper part of the spray mixer is hot, the drying of the moistening liquid begins in the upper part of the spray and mixer chamber immediately at and after the moistening of the material. The gas introduced into the spray mixer chamber from above absorbs in the spray and transition zone liquid, whereby the partial pressure of the liquid vapor in the gas is increased. Due to the fact, however, that the gas is introduced at high temperature, this partial pressure is not so high that the drying speed during the period of transition would thereby be substantially affected.
On the other hand, the invention is also based on the fact that it is advantageous to reduce the drying speed in the period of transition considerably or even to eliminate the drying in such zone, so that the liquid can penetrate the pores or interstices of the moistened material so much better and soaks the latter more uniformly, or that the liquid present on the surface of the material for a longer time facilitates the agglomeration process. In order to achieve this, the teaching of the invention consists in that the material, after the moistening with the liquid, is left during the period of transition in an atmosphere of high partial pressure of the liquid vapor. In practice this is accomplished by introducing, at the head of the spray mixer, gas of a low temperature and/or gas with high partial pressure of the moistening liquid vapor, and/or by enriching the gas in the spray area or in front thereof with vaporized moistening liquid in order to increase the partial pressure thereof.
A high partial pressure means a pressure close to the saturation pressure of the vaporized liquid. The invention can therefore also be expressed in such a manner that in the treatment zone the gas should have a relative humidity of close to 100%, with relation to the moistening liquid.
Suitably such an atmosphere with a relative humidity of close to 100% is present not only in the transition zone adjacent to the moistening zone, but in the moistening zone itself, so that the period during which the moistening liquid remains unevaporated is as long as possible, or so that the height of the spray mixer chamber can be correspondingly diminished by the shortening of the transition zone adjacent to the moistening zone.
In order to avoid a chilling of the material by a sudden transition from the saturated atmosphere of the transition zone into the drying zone of the fluidized bed, it may be practical to include an intermediate zone wherein the partial pressure of the moistening liquid vapor decreases gradually.